1. Field of the Invention
The invention disclosed in this specification and the like relates to a display device and an electronic appliance.
2. Description of the Related Art
In recent years, flat panel displays such as liquid crystal displays and organic EL displays have been widely used. In each of pixels provided in the row direction and the column direction in a display device such as a flat panel display, a transistor serving as a switching element, a display element electrically connected to the transistor, and a capacitor connected to the display element are provided.
As a silicon semiconductor film which is used for a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case where a display device is manufactured over a large substrate, a transistor using an amorphous silicon film, which can be formed using the established technique for forming a film over a large substrate, is preferable. Manufacturing the display device over the large substrate can reduce manufacturing costs of the display device. However, since a transistor using the amorphous silicon film has low field-effect mobility, the area of the transistor needs to be increased for sufficient on-state current. The aperture ratio of a pixel decreases as the area of the transistor increases, which results in an increase in power consumption of the display device.
In contrast, a transistor using a polycrystalline silicon film has high field-effect mobility; thus, sufficient on-state current can be obtained even when the transistor has a small area. Thus, the aperture ratio of a pixel can be increased, which results in a reduction in power consumption of a display device. However, the polycrystalline silicon film is formed by performing high-temperature heat treatment or laser light treatment on an amorphous silicon film, and thus is difficult to be formed over a large substrate. Since the display device is difficult to be manufactured over the large substrate, manufacturing costs of the display device is increased.
Oxides having semiconductor characteristics (also referred to as oxide semiconductors) are semiconductor materials that can be used for semiconductor films in transistors. For example, a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide is disclosed (see Patent Documents 1 and 2).
An oxide semiconductor film can be formed by a sputtering method, and thus is suitable for manufacturing a display device over a large substrate. Manufacturing the display device over the large substrate can reduce the manufacturing costs of the display device. A transistor using the oxide semiconductor film has high field-effect mobility; thus, sufficient on-state current can be obtained even when the transistor has a small area. Thus, the aperture ratio of a pixel can be increased, which results in a reduction in the power consumption of the display device. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
As the resolution of the display device increases, the area occupied by a wiring, an electrode, and the like increases; thus, the aperture ratio of a pixel is reduced, which results in an increase in the power consumption of the display device. For example, in the case where the width of a wiring is reduced in order to increase the aperture ratio of a pixel, the operation of the display device is delayed, which results in deterioration of the display quality of the display device in some cases. Also in the case where the size of a capacitor is reduced in order to increase the aperture ratio of a pixel, deterioration of the display quality of the display device is caused in some cases.
An oxide semiconductor film is known to transmit visible light because of its energy gap as wide as approximately 3 eV to 4 eV. Patent Document 3 discloses that in a display device, a channel layer of a transistor and one capacitor electrode used for a capacitor are formed on the same surface as a light-transmitting oxide semiconductor film. The other capacitor electrode used for the capacitor is formed of a light-transmitting pixel electrode; thus, the capacitor as a whole can be transparent.    [Patent Document 1] Japanese Published Patent Application No. 2007-123861    [Patent Document 2] Japanese Published Patent Application No. 2007-96055    [Patent Document 3] U.S. Pat. No. 8,102,476